1995 Nitro Tracker 115 Tachometer Troubleshooting.

I have a 1995 Nitro 180 FS w/ a Mercury Tracker 115 two stroke and my tachometer is not working. I was troubleshooting it yesterday and was getting 12v at the guage(purple and black) but only .3v from the pulse (grey wire) which I know I should be getting 5v at. The guage zeros out when I turn the ignition but doesn't move at all once the motors running. I installed a brand new guage and got the same results so I ruled out that the tachometer itself is not the problem. When I put my meter on the cranking battery with the motor running I was getting very sporadic readings from 13v-18v which I'm not sure is normal as when you put a meter on a running car battery it's usually a steady reading indicating the alternator is working. The motor is not running smooth as it should right now because the carbs still have some fogging oil in them so not sure if that may be why the reading weres so sporadic. I'm not sure what next step I should be taking in figuring out this problem any help would be greatly appreciated.